Thin-film spectrometers comprising a stratified film including a light-guiding layer and an echelon or staircase grating can be implemented in two basic grating variations, as disclosed in the European patent document 0 250 824 A1. Such staircase gratings can be provided as a reflection grating, as shown in FIG. 1 of the aforementioned EP document, or as a transmission grating, as shown in FIGS. 3 through 6 of the aforementioned EP document.
FIG. 1 of the EP document shows that a reflection grating offers the advantage that it can be made self-focusing by its overall curvature. A transmission grating, on the other hand, requires separate focusing means, as shown in WO 97/27 460. As shown in the WO document, a transmission grating offers the substantial advantage over a reflection grating that, once the indices of refraction in the light-guiding layer and in the second medium have been appropriately selected, the step height at the same spectrometer wavelength resolution can be larger the grating may be manufactured in relatively economic and accurate manner.
In the state of the art, the gratings of both types are designed with rigorously constant step sizes.
Moreover focusing echelon or staircase gratings, also called just a xe2x80x9cechelons xe2x80x9d, beyond the above species are known wherein. the steps change continuously in the direction the staircase. Illustratively such staircase gratings are part of the Fresnel zone plate as ilustratively discussed in
GRIMSEHLS LEHRBUCH DER PHYSIK, 1944, Teubner, p 387.
A more recent application of this principle is found in the article
Collimating cylindrical diffractive lenses:
rigorous electromagnetic analysis and scalar approximation
Elias N Glytsis
APPLIED OPTICS, vol. 37, #1, Jan. 1, 1998, pp 34-43.
In this design the steps are of constant height. Only the step width varies.
All grating designs based on the Fresnel zone principle share the feature they can only reproduce a single wavelength at one focal point. No imaging takes place at other wavelengths and consequently this design is inapplicable to a spectrometer and hence outside the species being considered.
Another grating design beyond the species is known from the article,
Diffractive phase elements that implement wavelength demultiplexing and spatial annular focusing simultaneously,
Bi-Zhen Dong et al
J. Opt. Soc. Am. A, vol. 14, #1, January 1997, pp 44-48.
In this grating the line spacings are constant and the step heights are configured in a complex, wholly irregular topography which, by means of a very complex calculation, determines focusing conditions for different wavelengths only approximately. Such a height topography is shown therein as FIG. 2a. FIG. 2 shows the spectrum with the diffraction errors (spurious lines). This design is manifestly unsuited for practical applications.
An object of the present invention is to create a spectrometer of the above indicated species which comprises a transmission grating that focuses the spectrum in simple manner.
The present invention is further directed toward a spectrometer having a light guide layer and a line sensor, the light guide layer being sandwiched between two cover layers. The cover layers have a lower index of refraction as compared to the light guide layer. An end surface of the light guide layer having an end surface fitted with a grating upon which light passing through the light guide layer is incident. The line sensor has imaged thereon a spectrum. The staircase grating is fitted with transparent step surfaces and with step edges perpendicular to the plane of the layer. The grating""s step surfaces form an interface with a second medium having an index of refraction that is less than the index of refraction of the light guide layer.
The height and the width of the grating continuously changes in the staircase direction such that, for two selected wave lengths, the total optical paths through the light guide layer and the second medium as far as one of the focal points pertaining to its particular wavelength shall differ for all adjacent front surfaces of the grating by a length difference which is a multiple of this very wave length.
The individual, mutually similar but nevertheless ever smaller steps for the grating of the invention can be computed sequentially step by step in serial calculation stages. The required number of steps, for instance one thousand, can be calculated in a very short time by a computer. In this procedure the light paths (geometric path multiplied by the index of refraction of the particular medium) to the grating and from the grating to the focal point can be computed each time for two adjacent front surfaces of the grating. These computations are carried out for two selected wavelengths within the spectrum being analyzed with previously determined focal points for each wavelength.
Constructive interference takes place for each wavelength and for two adjacent front surfaces at one step at the focal point belonging to the particular wavelength. The interference of one front surface with the light of the next front surfaces in one direction will be constructive interference with a light path difference which is a higher multiple of the wavelength. As a result, not only does such a grating focus at the two focal points for the two selected wavelengths, but it will also focus intermediate wavelengths and wavelengths outside this interval at focal points which, except for minute and unobjectionable deviations, shall be situated in a focal plane determined by the two selected focal points. A spectrometer of this kind will image the spectrum itself while using a transmission grating and while retaining the grating""s advantageous properties regarding allowable, increased step heights, and additional focusing means, therefore, are not needed. In this manner the design is considerably simplified relative to known spectrometers using transmission ratings and with additional focusing means.
In accordance with further features of the invention, the spectrometer""s stratification ends at the echelon grating, and as a result layer manufacture can be simplified.
In further accordance with an alternative embodiment of the present invention, the second medium is constituted by a light guiding layer on the other side of the staircase rating. According to this embodiment, more options are offered by selecting the media of the first and second light guide layers so as to attain an advantageous and very small difference between the indices of refraction adjoining the echelon, whereby, and as elucidated in WO 97/27 460, larger step heights are possible at a given wavelength resolution.